﻿Hi how you doing? I’m going to teach you how to make sense of Feynman diagrams,
know what all the squiggly lines mean, and work out if they are representing
a valid particle interaction or if they are defying the laws of physics. Ugh.
A Feynman diagram is a picture of what happens when particles interact with each other.
Particles go in one end, come out the other, and some stuff happens
in the middle. And to make sense of Feynman diagrams you need to know three things.
What all the lines and arrows mean. What all the particles in the standard
model are as well as their special properties like charge and spin.
What the overall rules are. These are called conservation rules, but you could call them
in-out rules, as in, you can’t get out a different amount out than what you put in.
Here’s a Feynman diagram. The lines represent different particles which are travelling through
space and time which are represented by these axes. Note that with one dimension of space
we are losing some spatial information here so these really are schematics of what a particle
interaction looks like rather than an accurate depiction of our four dimensional reality.
Here’s an example with a little animation to go with it. Two electrons move towards each other,
exchange a photon, and move away from each other again,
and that’s what this looks like as a Feynman diagram.
In this video I’m drawing time going upwards, but some people draw it going sideways,
which is quite annoying. So, the first thing when you look at a Feynman diagram is to check
which way time is going, hopefully they tell you and if they don’t they are bad people.
Drawing time going upwards is the most common way so that’s what I’m doing here.
Okay that’s the basics, now let’s go onto part one of the three things we need to know.
What are the squiggly lines. These are the four kinds of lines,
each corresponding to a kind of particle. And I can only explain this if I show you
all the particles, so here they are. These are the fundamental particles of the
Universe that everything is made of (as far as we know so far).
But why are they organised like this instead of being organised like this? Ugh.
It’s because some particles share similar properties so we can group them together.
Kind of like my cheese collection. The straight lines represent the fermions
which are these particles. These other particles are all called bosons. Now the difference between
fermions and bosons at a basic level is the fermions make up the stuff in the universe and
the bosons are how the stuff interacts with each other and they are called force carriers.
In fact, everything we experience is made of just these three fermions: the up and down quarks
which make protons and neutrons, and when you add electrons to protons and neutrons you get atoms,
that we’re made of and everything is made of and you’re made of.
The bosons, the force carries, are all the other lines.
We use a squiggly line for the photon, which is the force carrier for the electromagnetic field,
The spiral lines are for gluons which carry the strong force that keeps the nucleus of atoms
together and interacts only with these specific fermions which are called the quarks.
Now onto the weak force. The mediators of the weak force are the W and Z bosons and are represented
with a dashed line. All particles interact with the weak force apart from the gluons.
If this is all a bit much to take in don’t worry I’ve made a companion poster which you can study
in your own time the link in below. Finally there is the Higgs boson which we
also use a dashed line for. And those are all the lines. What about gravity the fourth fundamental
force?. Haha,
we don’t have gravity here. This is particle physics!
You’re supposed to ask about the arrows. But what about the arrows? Thanks for asking!
These only appear on the solid lines of the fermions, and they point forwards in
time for matter particles and backwards in time for anti-matter particles.
Because weirdly, in particle physics, an antimatter particle is exactly the same
as a matter particle that’s travelling backwards in time. But where on my little picture are the
antimatter particles? Oh yeah, nobody ever bothers to draw them on the standard model,
so I’ve done it here, here they all are. There are antimatter copies of all these
particles, which have the same mass as the matter versions, but opposite everything else.
And while we’re actually drawing actually all the particles I may as well draw the different colour
charges of the quarks and the eight kinds of gluons. These colours aren’t real by the way just
our labelling system for keeping track of the rules of the strong force. So this is
actually all of the fundamental particles. Goodness, there are a lot of rules aren’t there?
I’m sorry it’s all so complicated, but that’s just life isn’t it?
So now we’ve covered part one and part two: what the squiggly lines mean and what all the particles
are. This is enough to be able to read Feynman diagrams and make sense of what is going on.
So let’s apply this knowledge with some examples.
Here’s one we met earlier showing two electrons interacting with a photon and is an example of
an interaction via the electromagnetic force. Here are a couple more but these
are interactions involving the weak force and here’s a couple involving the strong force.
You’ll notice that these are all a similar shape, and I did that on purpose because I want to talk
about vertices. If you chop these in half you get a vertex, which is basically a meeting point
between three particles, and you can think of these vertices as the building blocks of Feynman
diagrams. You can build most feynman diagrams out of this basis set of vertices, which I’m not
going to describe in detail now, but pause the video now if you want to study them more.
What is interesting about these is that they mean different things depending on how they
are oriented in space and time. For example, this is an electron that has a change of momentum when
it emits a photon. But if I rotate it this electron becomes a positron, remember that
rule about matter travelling backwards in time is just antimatter travelling forwards in time. And
then this shows a positron and an electron coming together to annihilate into a photon. So different
orientations can mean very different things. So with this basis set of vertices put in
different orientations, you’ve got pretty much all you need to build any feynman diagram.
Let’s look at some more complicated examples. Here are the electrons interacting again, but
in the middle the photon spontaneously turns into an electron positron pair,
before annihilating and turning into a photon again.
See if you can work out what’s going on here? This is a neutron, which is made of three quarks: up,
down, down. It decays into a proton via the weak force. A proton is made of up, down, up quarks.
The w boson that is emitted then decays into an electron and an anti-electron neutrino.
So this is how neutrons can turn into protons via the weak force and is known in nuclear
physics as a beta decay emission. This last one I’m going to leave to you
to disentangle. See if you can work out what is going on here and specifically can you answer
which force is mediating this interaction? Let me know in the comments below what you think..
Now you know how to read Feynman diagrams, congratulations! But, we don’t yet know how
to analyse them to see if they are valid or not. For that we need to learn the underlying rules the
particles follow called the conservation rules of particle physics, but to avoid
information overload I’m going to cover that in my next video, so stay tuned for that.
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Thanks for watching and I’ll see you soon to delve deeper into this poster
of the standard model where we’ll lear the third part of feynman diagrams where we
understand the underlying rules that the fundamental particles follow. See you then.